1. Field of the Invention
The present invention relates to an imaging-control apparatus controlling a sensor configured to detect an applied radiation as an electrical charge, a method of controlling the imaging-control apparatus, and a program provided to make a computer execute the control method.
2. Description of the Related Art
Imaging apparatuses are known that include amorphous silicon and/or polysilicon provided on a glass substrate as a film and that use a sensor including pixels arranged in a two-dimensional manner, where each of the pixels includes a photoelectric-conversion element and a thin-film transistor (TFT). Usually, the above-described image-capturing apparatuses perform matrix driving by using the TFTs, so as to read an electrical charge obtained through photoelectric conversion performed by each of the photoelectric-conversion elements.
Further, for example, X-ray-image-capturing apparatuses (radiography apparatuses) configured to capture the X-ray image of a subject by irradiating the subject with an X ray, which is one type of radiation, and detecting the X ray that passed the subject have been available, for example in the medical field. According to the X-ray-image-capturing apparatuses, the X ray that passed the subject is converted into visible light through a phosphor provided on each of the pixels of the sensor and the visible light is made incident on each of the pixels of the sensor (refer to Japanese Patent Laid-Open No. 2003-244557 and Japanese Patent Laid-Open No. 2003-78124, for example).
Japanese Patent Laid-Open No. 2003-244557 discloses a technology of performing a so-called offset correction when an X-ray image is captured by reading electrical charges of the photoelectric-conversion elements.
Japanese Patent Laid-Open No. 2003-78124 discloses an X-ray-image-capturing apparatus that can capture not only a still image but also video.
Hereinafter, reading of electrical charges of photoelectric-conversion elements of a sensor provided in an X-ray-image-capturing apparatus configured to capture video will be described.
FIG. 10 is a schematic plan view of a sensor 4 including pixels arranged in a two-dimensional manner, where each of the pixels includes a photoelectric-conversion element and a TFT. Further, FIG. 10 shows an upper-half area 4u and a lower-half area 41 of the sensor 4, and a position P11 of the upper end of the sensor 4, a position P12 of the center part of the sensor 4, and a position P13 of the lower end of the sensor 4.
The time period where electrical charges are read can be reduced by performing the electrical-charge reading in the upper-half area 4u and the lower-half area 41 of the sensor 4 at the same time. Here, each of arrows 101 and 102 indicates a direction in which the electrical charges are read in the above-described example. In that case, the electrical-charge reading is performed from the upper-end position P11 toward the center-position P12 and from the lower-end position P13 toward the center-position P12 at the same time.
FIG. 11, which shows a known example, is a timing chart showing example time where the X-ray irradiation is performed in video-capturing mode and example time where electrical charges are accumulated and/or read in and/or from the electric-conversion elements of the sensor in the video-capturing mode. According to the time shown in FIG. 11, each reading of electrical charges of the electric-conversion elements that had been irradiated with an X ray and reading of electrical charges of the electric-conversion elements irradiated with no X ray is performed a single time within the time period where the X-ray image corresponding to a single frame is captured (a frame time).
In FIG. 11, the X-ray-irradiation time is designated by reference characters Tx, the electrical-charge-reading time is designated by reference characters Tr, and a pixel value which is read when X-ray-irradiation is performed is designated by reference characters Vx, a pixel value read at the X-ray-non-irradiation time is designated by reference characters Vd, and the frame time corresponding to a single frame is designated by reference numeral Tf11, and the electrical-charge-accumulation time is designated by reference numeral Tw11.
Usually, so-called offset correction should be performed, so as to obtain an X-ray image. More specifically, the offset correction is performed by subtracting the pixel value Vd which is read when no X-ray irradiation is performed from the pixel value Vx read at the X-ray-irradiation time. Further, for thoroughly achieving the offset correction, the time period where electrical charges are accumulated while irradiation with the X ray is performed should be equivalent to the time period where electrical charges are accumulated when no X-ray irradiation is performed.
Since the X-ray irradiation should be performed while electrical charges are accumulated through the photoelectric-conversion elements, the electrical-charge reading is performed after the X-ray irradiation is finished. Further, the X-ray irradiation is performed after the electrical-charge reading performed without the X-ray irradiation is finished, where the electrical-charge reading is performed within the previous frame.
In that case, the time where electrical charges that had been irradiated with the X ray are read in the photoelectric-conversion elements provided at the upper-end position P11 and the lower-end position P13 of the sensor 4 shown in FIG. 10 corresponds to the first part of the time period where the electrical charges that had been irradiated with the X rays are read, as shown in FIG. 11. Further, the time where electrical charges that had been irradiated with the X rays are read in the photoelectric-conversion elements provided at the center position P12 of the sensor 4 corresponds to the last part of the time period where the electrical charges that had been irradiated with the X ray are read.
Incidentally, when a part-to-be-photographed of the subject is a part moving with speed, such as the heart, high-speed imaging with a small visual field is demanded. On the other hand, when the part-to-be-photographed of the subject is a part which hardly moves, such as the head, low-speed imaging with a large visual field is demanded. An X-ray-image-capturing apparatus achieved in consideration with the above-described demands is disclosed in Japanese Patent Laid-Open No. 08-47491, for example. More specifically, Japanese Patent Laid-Open No. 08-47491 disclosing the above-described X-ray-image-capturing apparatus further discloses a method of changing the read range of a sensor and driving an X-ray diaphragm according to the read range.
However, according to the X-ray-image-capturing apparatus shown in Japanese Patent Laid-Open No. 08-47491, the read range of the sensor should be controlled, so as to reduce the read range of the sensor for achieving the high-speed imaging with the small visual field, and to increase the read range of the sensor for achieving the low-speed imaging with the large visual field. Consequently, hardware relating to reading of the electrical charges of the sensor is complicated.